1. Field of the Invention
The present invention relates to a mobile communication method, and more specifically, it relates to a wireless receiver estimating the power of interference in order to control the transmitting power of a base station.
2. Description of the Related Art
Spread spectrum communication systems are widely used as a fundamental technology of mobile communications.
In a direct spread (DS) method as the simplest model of spread spectrum communication, a data signal to be transmitted is modulated using a PN signal with chip width Tc of approximately 1/100 to 1/1000 of its cycle (bit width) T as a spread signal, that is, its spectrum is spread by multiplying the data signal by the PN signal, and is transmitted to a receiver.
The receiver despreads the signal buried in noise and extracts symbols from the signal. To despread basically means to demodulate an incoming signal by multiplying the incoming signal by the same PN signal with the same phase as that of the incoming signal.
In code division multiplex access (CDMA), which is one typical spread spectrum communication method, since the signals of individual channels interfere with one another, a receiver can estimate the signal-to-interference power ratio (SIR) of an incoming signal and a base station can control the transmitting power, based on the result. In this case, channel capacity can be increased by communicating with the minimum necessary transmitting power. SIR estimation is a major technology of a CDMA system.
Specifically, according to the result of the SIR estimation of the receiver, if the signal power is sufficiently high compared to the power of interference, transmitting power is reduced by this technology in the base station. If it is low, transmitting power is increased. FIG. 1A shows the configuration of a conventional power measurement unit of a receiver conducting such a SIR estimation.
FIG. 1A shows the configuration of the level measurement unit of a rake receiver. A rake receiver combines signals received with a variety of delay times in a multi-path environment where there are many reflected waves besides direct waves transmitted from a transmitter. In such a receiver, conventionally, the SIR estimation of an incoming channel, that is, a data channel, is conducted in order to control its transmitting power in the base station, using its individual pilot signal.
In FIG. 1A, the resulting multi-path signals combined by a rake combination unit 70 are input to a pilot pattern cancellation unit 71 through a switch 80 that is turned on only when a pilot signal is output, and its pilot pattern is cancelled. Then, the power of a core signal, the interference element of which is eliminated by a voltage averaging unit 72 and a power conversion unit 73, that is, a signal to be obtained by despreading is obtained.
The signal input from the switch 80 is simultaneously converted into power by a power conversion unit 75 and a power averaging unit 76, the average power of a plurality of symbols is calculated, and the respective power of the core signal and interference element are totaled. Then, a subtracter 74 calculates the power of interference by subtracting the output of the power conversion unit 73 from the output of the power averaging unit 76. Then, the power of interference is input to an SIR calculation unit 77 together with the core signal power, being the output of the power conversion unit 73, and an SIR is estimated using the core signal power and the power of interference.
However, as described with reference to FIG. 1A, according to the conventional method for estimating a SIR using the data channel's pilot signal, if the number of pilot symbols in the slot of an individual data channel is small, its interference element cannot be thoroughly eliminated by simply averaging the pilot symbols, and accordingly, the power of the core signal cannot be accurately obtained, which is a problem.
FIG. 1B shows this problem. FIG. 1B shows an example of a signal point obtained by canceling the pilot pattern if there are only two pilot symbols in one slot.
The first and second symbols of a measured signal point are located at places marked with a circle and the true signal point is marked with × on the phase plane of QPSK. When a signal point is calculated by averaging these points, it is located away from the true signal point. In particular, if the number of pilot symbols is one, an average cannot be obtained, and the power of the core signal cannot be obtained by simply using pilot symbols, accordingly.
If the number of pilot symbols is small, its data can be temporarily judged by the prior art, and the respective power of the core signal and the power of interference can also be obtained by increasing the number of symbols to be averaged. If interference is weak and the result of the temporary judgment is accurate, it is valid. However, if interference is strong, the result of the temporary judgment is inaccurate, and accurate core signal power and the power of interference cannot be obtained. In this way, if the number of pilot symbols in the slot of a data signal is small, a correct SIR cannot be estimated, which is another problem.